An optical waveguide is formed on a substrate such as silicon substrate to have a predetermined shape for transmitting light wave in the waveguide. Therefore, the waveguide is similar to an electric wiring for flowing current. The waveguide is provided by an optical device. The device includes a silicon substrate, a cladding layer on the substrate, and a core layer as an optical waveguide. The core layer is disposed on the substrate through the cladding layer. The cladding layer has low refraction index, and the core layer has high refraction index.
The core layer is made of SiO2 having high refraction index, and the cladding layer is made of SiO2 having low refraction index. The deposition of a SiO2 layer as the core layer has a limitation so that the thickness of the SiO2 layer is limited. Therefore, connection loss at a connection between the waveguide and an optical source becomes larger.